The ontouchend attribute is not standardized. It is even not widely supported. This means you can use it in some browsers, but validators will throw warnings.

You can simply ignore it, when it works for you. But this don't mean that this attribute behavior meight not change in the future.

Quotes:

This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for the proper prefixes to use in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the spec changes.

Note: This attribute has not been formally standardized. It is specified in the Touch Events – Level 2 Editor's Draft specification and not in Touch Events Recommendation. This attribute is not widely implemented.